Khaled al-Ayoubi’s resignation made him one of the most prominent Syrian diplomats to turn their back on the Syrian president during the 16-month rebellion that has claimed more than 15,000 lives. He has not yet defected - officially switched his allegiance to the Syrian opposition - although is understood to be considering his next move.

A statement from the Foreign Office said Mr Ayoubi had informed it of his decision on Monday morning.

It read: “Mr al-Ayoubi has told us that he is no longer willing to represent a regime that has committed such violent and oppressive acts against its own people, and is therefore unable to continue in his position.

“Mr al-Ayoubi was the most senior Syrian diplomat serving in London. His departure is another blow to the Assad regime. It illustrates the revulsion and despair the regime’s actions are provoking amongst Syrians from all walks of life, inside the country and abroad.

“We urge others around Bashar Al-Assad to follow Mr al-Ayoubi’s example; to disassociate themselves from the crimes being committed against the Syrian people and to support a peaceful and free future for Syria.”

He is understood to be in a safe location in Britain with his immediate family.

Mr al-Ayoubi was born in 1971 and joined the Syrian diplomatic service in 2001. His first posting was in Greece as consul from 2003 – 2008. He joined the London embassy as second secretary in February 2011, a month before the first protests against Mr Assad. It is presumed he served back in Damascus in the three years between the foreign postings.

His resignation followed a series of defections by senior Syrian officials in recent weeks, including diplomats in several countries, including the ambassador to Iraq, and top army officers.

Just five Syrians of junior rank now remain at the London embassy in Belgrave Square. Mr al-Ayoubi’s predecessor was expelled in May with two colleagues as part of a coordinated series of expulsions by Western nations in response to a massacre committed by Syrian forces in Houla.

Mr Assad’s regime withdrew Syria’s ambassador from London in March in retaliation for the decision by Britain, the United States and several European countries to recall their ambassadors or close their embassies in Damascus due to security concerns.

Britons in difficulty in Syria are recommended on the Foreign Office website to contact the Hungarian embassy.

Earlier on Monday it was reported that the deputy police chief in the western Syrian city of Latakia was among 12 officers who had fled across the border to Turkey.

France, which is taking over the United Nations Security Council's rotating presidency in August, will call an emergency ministerial meeting on Syria in a bid to end the diplomatic stalemate.

Laurent Fabius, the French foreign minister, said: "We must try everything" even though Russia and China have blocked three resolutions on Syria.

"We cannot say that it is a domestic matter," he added, warning that the conflict could spread into neighbouring countries.

Mr Fabius also expressed fears of a massacre in Syria's most populous city Aleppo, where the government has launched a fierce offensive against rebels since Saturday.

"The Syrian people are being martyred and the executioner is called Bashar Al-Assad," he said.

Syrian troops continued their assault on Aleppo, with shells and machine guns.

The early morning bombardment was focused on the southwestern district of Salaheddin, a stronghold of the Free Syrian Army, made up of deserters and armed civilians, said the Syrian Revolution General Commission.

The army said later that it had overrun Salaheddin, although the rebels denied the claim.

The United Nations estimates that around 200,000 civilians have fled the fighting in Aleppo and that many more are trapped.

Head of UN humanitarian operations, Valerie Amos, requested that relief organisations be granted "secure access" to the northern city.